s390x/css: factor out some generic code from virtio_ccw_device_realize()

A lot of what virtio_ccw_device_realize() does isn't specific to
virtio; it would apply to emulated CCW as well. Factor it out to make
it easier to implement emulated CCW devices later on.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

include/hw/s390x/css.h

@@ -17,6 +17,7 @@
 #include "hw/s390x/ioinst.h"
 
 /* Channel subsystem constants. */
+#define MAX_DEVNO 65535
 #define MAX_SCHID 65535
 #define MAX_SSID 3
 #define MAX_CSSID 254 /* 255 is reserved */
@@ -24,6 +25,8 @@
 
 #define MAX_CIWS 62
 
+#define VIRTUAL_CSSID 0xfe
+
 typedef struct CIW {
     uint8_t type;
     uint8_t command;
@@ -169,4 +172,19 @@ extern PropertyInfo css_devid_propinfo;
 #define DEFINE_PROP_CSS_DEV_ID(_n, _s, _f) \
     DEFINE_PROP(_n, _s, _f, css_devid_propinfo, CssDevId)
 
+/**
+ * Create a subchannel for the given bus id.
+ *
+ * If @p bus_id is valid, verify that it uses the virtual channel
+ * subsystem id and is not already in use, and find a free subchannel
+ * id for it. If @p bus_id is not valid, find a free subchannel id and
+ * device number across all subchannel sets. If either of the former
+ * actions succeed, allocate a subchannel structure, initialise it
+ * with the bus id, subchannel id and device number, register it with
+ * the CSS and return it. Otherwise return NULL.
+ *
+ * The caller becomes owner of the returned subchannel structure and
+ * is responsible for unregistering and freeing it.
+ */
+SubchDev *css_create_virtual_sch(CssDevId bus_id, Error **errp);
 #endif